At least two voltage networks, each representing a circuit area in a vehicle, are provided, in general, in modern vehicles in order to supply the different electrical energy consumers during the operation of the vehicle. One of these voltage networks may be an on-board voltage network, in which, supplied from a first battery used as a voltage source, electrical energy consumers are supplied, which are to be supplied with electrical energy during the operation of the vehicle permanently and regardless of whether an internal combustion engine is operating or not. Another of these voltage networks may be a start voltage network, which is used primarily to also restart the internal combustion engine in a start-stop operation of an internal combustion engine after the internal combustion engine had been temporarily switched off, for example, with the vehicle stopped at a traffic light or during a coasting operation.
A circuit breaker device, which connects the two voltage networks or the voltage sources thereof to one another in a normal operating state, which does not lead to an overload especially of the on-board voltage network, is provided in association with such voltage networks, so that both voltage sources can be used and also recharged. In a state in which one of the voltage sources has a high load and thus a high voltage drop could also develop in the other voltage network, the two voltage networks can be disconnected from one another by the circuit breaker device. This may happen in the above-described example when the starting voltage network has a high load during the start of the internal combustion engine, but a load leading to an excessive voltage drop of the on-board voltage network must be ruled out.